1. Field of the Disclosure
This specification relates to an organic light-emitting diode (OLED) display device and a method of fabricating the same, and more particularly, a top emission type OLED display device and a method of fabricating the same.
2. Background of the Disclosure
As the interest in information display and demand on the use of portable information media increase, research efforts and commercialization are focusing mainly on display devices such as flat panel displays (FPDs) which are light in weight and thin in thickness.
Specifically, among such displays, liquid crystal display (LCD) devices have seen widespread use in view of its light weight and low power consumption.
As another display device, an organic light-emitting diode (OLED) display device emits light by itself and thus exhibits more excellent viewing angle and contrast ratio than the LCD device. Also, the OLED display device does not need a backlight and thus is advantageous in reduced weight, thickness and power consumption. The OLED display device is also current driven based upon a direct-current (DC) low voltage and exhibits a fast response speed.
Hereinafter, the basic structure and operation characteristics of an OLED display device will be described with reference to the drawings.
FIG. 1 is a diagram illustrating a light-emitting principle of a typical OLED.
An OLED display device typically includes an OLED having a structure as illustrated in FIG. 1.
Referring to FIG. 1, the OLED includes an anode 18 as a pixel electrode, a cathode 28 as a common electrode, and an organic compound layer(s) 31, 32, 35, 36 and 37 formed between the anode 18 and the cathode 28.
Here, the organic compound layer(s) 31, 32, 35, 36 and 37 includes a hole injection layer 31, a hole transport layer 32, an emission layer 35, an electron transport layer 36, and an electron injection layer 37. All or some of these layers may be separate or some may be combined together such that one layer is implemented with multiple functions.
With the configuration of the OLED, when positive (+) and negative (−) driving voltages are applied to the anode 18 and the cathode 28, respectively, holes passed through the hole transport layer 32 and electrons passed through the electron transport layer 36 are moved to the emission layer 35 to form excitons. When the excitons transit from an excited state into a base state, namely, a stable state, light with a predetermined wavelength is emitted.
In the OLED display device, sub pixels each having the OLED with the aforementioned structure are arranged in the matrix configuration. The sub pixels are selectively controlled by a data voltage and a scan voltage, to display various colors that collectively form an image.
Here, the OLED display device can be categorized into a passive matrix type and an active matrix type using thin film transistors (TFT) as switching elements. Of those types, in the active matrix type OLED display device, the TFT as an active element is selectively turned on to select a sub pixel and light emission of the sub pixel occurs due to voltage charged in a storage capacitor.
The OLED display device may also be categorized according to a light-emitting direction into a top emission type, a bottom emission type and a dual emission type.
The top emission type OLED display device is configured to emit light in a direction away from a substrate on which the sub pixels are arranged. The top emission type OLED display device is advantageous in that aperture ratio is greater than that of a bottom emission type in which light is emitted toward and through the substrate with the sub pixels arranged thereon.
The top emission type OLED display device includes an anode provided below an organic compound layer and a cathode provided above the organic compound layer through which light is transmitted.
Here, the cathode should be formed thin (˜100 Å) enough to be implemented as a semi-transparent layer with a low work function. However, doing so causes the cathode to have high resistance.
The thusly-configured top emission type OLED display device results in formation of a voltage drop (IR drop) by high specific resistance of the cathode. Accordingly, undesirably different levels of voltages are applied to sub pixels, respectively. This brings about non-uniform luminance or image quality. Specifically, as a size of a panel increases more, the voltage drop (IR drop) problem may become aggravated.
The OLED display device can be divided into a display area and a pad area located outside the display area.
The TFT and the OLED are formed in the display area.
A pad electrode is formed in the pad area to apply a signal voltage from an external power source to the TFT and the OLED.
In this case, pad electrodes formed in the pad area may suffer from corrosion due to moisture and oxygen. In addition, the corrosion may be caused by a specific etchant with respect to the material constituting the pad electrode. When the pad electrode corrodes, the transmission of signals is problematic, and thus the overall device reliability decreases.